Acid3, which was finalised earlier this month by the Web Standards Project, checks how closely a browser follows certain standards, particularly specifications for Web 2.0 applications, as well as standards related to the DOM (Document Object Model), CSS2 (Cascading Style Sheets) and SVG (Scalable Vector Graphics).

A Mozilla executive called the Acid3 test "a puzzle game" and said Mozilla would press ahead with work on Firefox 3.0 rather than devote resources to improving its Acid3 score.

On Wednesday Opera was the first to announce that an unreleased version of its browser had received a score of 100 - out of a possible 100 - on the Acid3 test. "Today we reached a 100 percent pass rate for the first time" said Lars Erik Bolstad, who heads the core technology group at Opera, in a post to a company-supported blog.

"There are some remaining issues yet to be fixed, but we hope to have those sorted out shortly."

Opera has not issued a public version of the development edition that passed Acid3, although Bolstad said one would be made available on the Opera Labs site "within the next week or so".

WebKit meanwhile, matched Opera Wednesday, then upped the ante. Late Wednesday, developers working on WebKit, the open-source project that provides the core engine for Apple's Safari browser, not only claimed that their newest build scored 100 out of 100 on Acid3, but released Mac OS X and Windows versions so users could see for themselves.

"WebKit has become the first publicly available rendering engine to achieve 100/100 on Acid3," said Maciej Stachowiak, a Polish developer on the WebKit team, in a post to the group's blog. In an update, Stachowiak claimed that additional tweaks had been made to the engine.

"We believe we have a pixel-perfect match for the reference rendering," he said in the update.

The newest Mac OS X and Windows builds can be downloaded from the WebKit site.

Mozilla's Mike Shaver, the company's chief evangelist, however, essentially said Firefox is taking a pass on Acid3 for the moment.

"We're almost at the end of the development cycle [for Firefox 3], and I don't want to see people pressuring us into making changes simply for Acid3, or us looking for shortcuts to get a better score," said Shaver in an interview.

In a posting to his blog earlier in the day, Shaver was more specific.

"We don't want to be rushing fixes in, or rushing out a release, only to find that we've broken important sites or regressed previous standards support, or worse, introduced a security problem," he said. Firefox 3, currently in beta, will likely ship in final form by the end of June, according to Mozilla.

Shaver also criticised the new standards test.

"Acid 3, unlike its predecessors, is not about establishing a baseline of useful web capabilities," he said in the blog. "It's quite explicitly about making browser developers jump. I don't think it's worthless, but I think it could have been a lot more."

In a telephone conversation, he elaborated. "The bar was set high by Acid2, which I thought tested for the things that developers were really clamoring to use on the web.

"I wish Acid3 was more pragmatic."

Just because it doesn't pass Acid3 at the moment - doesn't mean it won't at some point, Shaver added.

"Standards do matter. In the long term, we want to have no standards bugs. But does Acid3 help us get there? I think it helps, but it's also sort of a distraction."

And when Firefox 3.0 ships, said Shaver, it will be "within a point or two" of any other production browser.

"There's a difference between what you can do in development and what you do when you ship," he said.